Electrode for electric discharge devices



June 21, 1938. E. LEMMERs El AL ELECTRODE FOR ELECTRIC DISCHARGE DEVICES Filed April 16. 1935 n 6 e E9 6 mb Q can tl; 2i v 8A eM U ur m EM sive material on its surface.

Patented June 21, less pmrso srarss rarest errica ELECTRODE FOR ELECTRIC DISCHARGE DEWCES New York Application April 16, 1935, Serial No. 16,614

12 Claims. (0]. 176-1263) Our invention relates to electric discharge devices and particularly to electrodes therefor. Still more particularly our invention relates to electrodes of the type comprising a material which is electron emissive.

Electrodes have been produced heretofore by coating the surface of a metal tube, such as nickel, with an electron emissive material such as barium oxide. a disadvantage of such an electrode is the inability to retain very much of the electron emis- Electrodes have also been made by sintering a mixture of a powdered refractory metal such as tungsten and an electron emissive material such as barium oxide. This type of electrode, although effective and having a long life, has a very high specific resistance because of the separation of the tungsten particles by the barium oxide, and it is diincult to make and hard to handle.

One of the objects of our invention is to provide an electrode which is highly electron emissive and remains so for a long time. Another object is to provide an electrode which is much less expensive and easier to manufacture than those in use at the present time.

According to our invention, the electrode comprises a body of highly porous refractory metal, such as tungsten or molybdenum, which is impregnated with an electron emissive material.

Further features and advantages of our invention will appear from the following description of species thereof.

The drawing is an elevation of one form of discharge devlce in which electrodes made accord-' ing to our invention may be used, the device illustrated being a high pressure positive column metal vapor (mercury) arc lamp of the type disclosed in application Serial No. 8286, Eugene Lemmers, filed February 26, 1935. This lamp comprises a substantially oval-shaped glass envelope it con taining a readily ionizable gas, such as argon, and a vaporizable metal, preferably a globule of mercury H. A pair of electrodes i2, i3, made according to our invention as hereinafter described, are disposed in the vertical and longitudinal axis of said envelope, the upper electrode it being preferably disposed opposite the largest diameter of said envelope and the lower electrode is being disposed substantially adjacent the lower end of said envelope. The upper electrode i2 is mounted on a lead wire it which extends through an axial hole in said electrode. An angular piece of wire 85 is spct-welded to the said lead i and electrode i2 to secure the electrode on the lead. The lower electrode 53 is similarly mounted on a lead wire it and secured by an'angular wire i'i. The envelope i6 is so shaped and the electrodes are so mounted that the temperature of the envelope during operation is as nearly uniform as possible and very high temperatures and metal vapor pressures may be attained without bulging out or bursting the said envelope.

The first electrodes were made of porous pressed tungsten powder similar to that used as a starting material for making filaments. These were made by pressing the powder, which was of such size as to pass through a 200 mesh sieve, into the desired shape and then firing in hydrogen to form a slug or body having sufficient strength to be handled. These bodies were not really sintered or treated by passing an electric current therethrough and heating them nearly to the melting point as is done in a subsequent step before making the regular tungsten ingot for filament making purposes. The density of these bodies was about 10.9 grams per cubic centimeter, the porosity was about 43 per cent (that is, -23 per cent air space in the body), and the resistivity was about 80X 10 ohm cm.

The tungsten body made in the manner described above was then impregnated with a compound such as molten barium hydrate which impregnated the body to a considerable depth because of its high porosity. Upon subsequent heating, for example by mounting a pair of bodies in the glass envelope id and causing an arc to strike therebetween, or by means of high frequency current, the barium hydrate was broken down, leaving the electron emissive barium oxide in the bodies i2, E3. The water vapor component was drawn from the envelope by evacuation.

Although electrodes made in the manner described above are usable, they are rather chalky, or structurally weak, and the resistivity is high. If the tungsten body thus made is treated at close to its melting point by passing a current therethrough, its density is about I? to 18 which is not satisfactory because it is too difficult to impregnate and it is also difiicult to drive out the gaseous products therein.

A specific example of the preferred method of making the electrodes is by preparing a tungsten body in the following manner:

The first step is the preparation of a coarse tungsten powder. Ammonium tungstate crystals are reduced at 1000 to 1200 C. in a. low current of hydrogen gas. The product thus produced is completely reduced tungsten powder of varying degrees of fineness. This product is first put through a number mesh sieve to remove any scale and the sieved product is then put through a number 150 mesh sieve.

' sintering and treating which is capable ofbeing swaged and'drawn.

The coarse tungsten powder prepared in the manner described above is pressed and sintered to form an ingot which is then treated in hydrogen at about 92 per cent of the'fusing amperes for this material. The resulting product is a highly porous tungsten bar, or composite, of fair strength which can be machined and drilled. This material because of its high porosity, (over 30 per cent), may be termed a tungsten sponge. It is sufliciently porous to permit the vpassage of air or gas through it. It is fairly coherent and conductive, having a resistivity of about 9 10- ohm cm. Its resistance is therefore about 1.8 times the resistance of drawn tungsten wire. It has a density of about 12.7 as compared with a density of 17 to 18 for treated bars made from the regular tungsten powder used for making drawn wire filaments. This material may be cut up into pellets or slugs or it may be made in the form of tubes or any other desired shape.

The highly porous tungsten bodies thus prepared may be impregnated with an electron emissive material,for example, by soaking them in various compounds which upon subsequent heat treatment yield an electron emissive compound. Thus, the porous tungsten bodies may be mounted on the lead wires l4, l6 and dipped in an alkaline earth compound such as molten barium hydroxide which impregnates the bodies to a considerable depth. The bodies may then be sealed in the glass envelope ill of the electric arc lamp or other discharge device and heated,-for example by causing an arc discharge to strike therebetw'een, or by means of high frequency. The barium hydroxide breaks down into barium oxide, which remains in the tungsten bodies l2, I3, and water vapor, which is removed from the envelope by'evacuation. Various other compounds may be used, some of which impregnate the porous tungsten body in molten condition, others from solution or. suspension, and' some from either state. Besides the hydroxide. other barium compounds which may be used are the nitrate, nitrite, iodide, formate, oxalate, acetate, and carbonate or other suspensions. The corresponding strontium compounds may be used withbarium compounds if desired. The body may also be impregnated in other ways'such as by diffusion of the material therethrough in a powder or vapor state.

It appears that in order to be satisfactory a porous tungsten body should have a density between the limits of ten and sixteen. If the density is over sixteen, the body is difilcult toimpregnate and it is difficult to drive out the gaseous products because of the small pores.

: On the other hand, if the density is less than ten, the body is chalky or structurally weak, or it does not satisfactorily hold a large amount of the electron emissive material. The preferable density is about 13 as obtained by the preferred method of making the body described above.

The use of the highly porous metal bodies results in a high absorption of electron emissive The fine particles pass through this sieve and the desired product is the material rather than a very thin coatingof the material on a non-porous metal surface such as used heretofore. The porous body may'also be made of molybdenum by preparing an ingot from coarse molybdenum powder andimpregnating it with an alkaline earth compound which upon subsequent heat treatment yields an electron emissive material.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. The method of making an electrode for-electric discharge devices which comprises soaking a highly porous refractory metal body in a liquid comprising an alkaline earth compound capable of being converted to an electron emissive material and subsequently subjecting said porous metal body to heat to convert said alkaline earth compound to an electron emissive material.-

2. The method of making an electrode for electric discharge devices which comprises soaking a highly porous tungsten body in a liquidcomprising an alkaline earth compound capable of being converted to an electron emissive mateform an ingot, heating said ingot nearly to the' melting point in a reducing atmosphere to form a coherent and highly porous bar, shaping said bar to a desired form of body for an electrode, and then impregnating said body with an electron emissive material.

4. The method of making an electrode for eiectric discharge devices which comprises preparing a coarse refractory metal powder of a fineness such that it passes through a forty mesh sieve but not through a one hundred and fifty mesh sieve, pressing and sintering said powder to form an ingot, heating said ingot nearly to the melting point in a reducing atmosphere to form a coherent and highly porous bar, shaping said bar to a desired form of body for an electrode, dipping said body in a liquid comprising an alkaline earth compound capable of being converted to an electron emissive material, and subsequently subjecting said body to heat to convert said alkaline earth compound to an electron emissive material.

5. The method of making an electrode for electric discharge devices which comprises prepar ing a coarse-tungsten powder of a fineness such 'said alkaline earth compound to an electron emissive material.

6. The method of making an electrode for electric discharge devices which comprises preparin'g a coarse refractory metal of a fineness such that it passes through a forty mesh sieve but not through a one hundred and fifty mesh sieve,

pressing and sintering said powder to form an ingot, heating said ingot nearly to the melting pointin a reducing atmosphere to form a coherent and highly porous body and then impregnating said body with an electron emissive material.

7. 'An electrode for electric discharge devices comprising a highly porous refractory metallic body consisting of strongly coherent coarse particles of the metal treated at a temperature just below the fusion point of the metal and having a porosity of at least thirty per cent, said body having a resistivity considerably less than that of a body of said particles sintered at about 1100-1300" C. and said body being impregnated with a material which is electron emissive.

- 8. An electrode for electric discharge devices comprising a highly porous refractory metallic body consisting of strongly coherent coarse particles of molybdenum treated at a temperature just below the fusion point-of the metal and having a porosity of at least thirty per cent, said body having a resistivity considerably less than that of a body of said particles sintered at about 1100-1300 CJ and said body being impregnated with a material which is electron emissive.

9. An electrode for electric discharge devices comprising a highly porous refractory metallic body consisting of strongly coherent coarse particles of tungsten treated at a temperature just below the fusion point of the metal and having a density of between ten and sixteen, said body having a resistivity considerably-less than that of said body of particles sintered at about 1100-1300" C. and said body being impregnated with a material which is electron emissive.

10. An electrode for electric discharge devices comprising a highly porous refractory metallic body consisting of strongly coherent coarse particles of tungsten treated at a temperature just below the fusion point of the metal and having a density of approximately thirteen, said body having a resistivity considerably less than that of said body of particles sintered at about 1100'-1300 C. and said body being impregnated with a material which is electron emissive. v

11. As an article of mannfacture, a highly porous refractory metallic body consisting of strongly coherent coarse particles of the metal treated at a temperature just below the fusion point of the metal and having a porosity of at least thirty per cent, said body having a resistivity considerably less than'that of said body of particles sintered at about 1100-1300" C.

12. As an article of manufacture, a highly porous refractory metallic body consisting of strongly coherent coarse particles of tungsten treated at a temperature just below the fusion point of the metal and having a density of between 10 and 16, said-body having a resistivity considerably less than that of said body of particles sintered at about 1100-1300 C;

EUGENE HARRY M. FERNBERGER. 

